1. Field of the Invention
The present invention relates to a solid electrolytic capacitor including an anode made of a valve action metal such as aluminum, tantalum or niobium, and also including a solid electrolyte such as a conductive polymer or manganese dioxide.
2. Description of the Prior Art
A typical solid electrolytic capacitor including a valve action metal for an anode is manufactured in the following manner. First, an anode is made of a porous compact of the valve action metal such as a surface-roughened aluminum foil or a porous compact of a sintered powder of a valve action metal such as tantalum or niobium. The entire surface of the porous compact of the valve action metal is covered with a dielectric oxide film. Secondly, a solid electrolytic layer of a conductive polymer such as polypyrrole or manganese dioxide is formed on the surface of the dielectric oxide film, and a cathodic layer of a carbon layer, a silver layer or the like is formed on the solid electrolytic layer. Subsequently, an anodic extraction terminal is attached to an anodic lead by welding or the like, and a cathodic extraction terminal is attached to a cathodic layer with a conductive adhesive or the like. Lastly, the entire element is covered with a packaging resin from which the cathodic extraction terminal and the anodic extraction terminal are partially exposed to the outside. Alternatively, the solid electrolytic layer can be connected electrically with the cathodic extraction terminal without forming a cathodic layer.
Since the packaging resin serves to maintain airtightness from the outside, it should be adhered securely to electrode extraction members at a part to extract terminals from the package. Especially, when the solid electrolyte is a conductive polymer, insufficient airtightness will cause considerable deterioration and thus, the electric properties will be difficult to maintain favorably for a long time. Therefore, for securing the airtightness, the packaging resin is formed generally from an epoxy-based thermosetting resin, using molding (a tip capacitor) or dipping (a lead capacitor).
A solid electrolytic capacitor having a packaging case is also known. Such a capacitor is manufactured by inserting an entire element in a case where a cathodic extraction terminal and an anodic extraction terminal are partially extracted outward, and by sealing the opening of the case with a resin or the like.
A typical solid electrolytic capacitor is mounted onto a substrate by soldering or the like to be used. Due to heating and cooling in the mounting, distortion occurs inside the solid electrolytic capacitor. This distortion is caused by stress due to differences in thermal expansion coefficients between members including the substrate, the solder and a temporary-tacking agent previously formed on the substrate, and the solid electrolytic capacitor, and also differences in thermal expansion coefficients between the respective members inside the solid electrolytic capacitor. A solid electrolytic capacitor of a case-inserting type as disclosed in JP-A-3-276621 can absorb distortion in the inner space. However, for a dipped product as disclosed in JP-A-5-243096 and a molded product as disclosed in JP-A-8-213285, the distortion due to the thermal stress will cause partial peeling inside the solid electrolytic capacitor.
This peeling occurs where the adhesive force (bonding force) is the poorest. In a conventional solid electrolytic capacitor, peeling often occurs in an interface between a solid electrolytic layer and a cathodic layer, or between a solid electrolytic layer and a cathodic extraction terminal. When peeling occurs, efficiency in cathode extraction is lowered and electric properties of the product will deteriorate.